1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing-plate constructions for on- or off-press imaging using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or xe2x80x9cfountainxe2x80x9d) solution to the plate prior to inking. The ink-repellent fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) describe an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a blank lithographic printing plate, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing plate and focused onto its surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the first surface layer).
U.S. Pat. Nos. 5,807,658, 5,783,364 (the ""364 patent), 5,339,737 (the ""737 patent), and Re. 35,512 (the ""512 patent), the entire disclosures of which are hereby incorporated by reference, describe a variety of lithographic plate configurations for use with such imaging apparatus. In general, the plate constructions may include a first, topmost layer chosen for its affinity for (or repulsion of) either ink or an ink-repellent fluid. Underlying the first layer is an imaging layer, which ablates in response to imaging (e.g., infrared, or xe2x80x9cIRxe2x80x9d) radiation. A strong, durable substrate underlies the imaging layer, and is characterized by an affinity for (or repulsion of) either ink or an ink-repellent fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging radiation pulse generally weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This creates an image spot having an affinity differing from that of the unexposed first layer, for either ink or an ink-repellent fluid, the pattern of such spots on a lithographic plate forming an image. Laser imageable materials may be imaged by pulses of near infrared (near IR) radiation from inexpensive solid-state lasers. Such materials typically exhibit a nonlinear response to near-IR exposure, namely, a relatively sharp imaging-fluence threshold for short-duration laser pulses, but essentially no response to visible light. A longstanding goal of plate designers is to increase responsiveness to imaging radiation while maintaining desirable properties such as durability, manufacturability, and internal compatibility.
One strategy frequently proposed in connection with photothermal materials is incorporation of energetic (e.g., self-oxidizing) compositions, which, in effect, contribute chemical energy to the imaging process. For example, the ""737 patent mentioned above discloses nitrocellulose layers that undergo energetic chemical decomposition in response to heating. Unfortunately, these materials do not help to concentrate radiation within the imaging layer or increase the efficiency of its utilization. Instead, they are either employed as essentially interchangeable alternatives to non-energetic materials, or as propellent layers in transfer-type materials (see, e.g., U.S. Pat. Nos. 5,308,737, 5,278,023, 5,156,938 and 5,171,650).
Other reported plate constructions, such as in U.S. Pat. No. 5,570,636 (the ""636 patent) the entire disclosure of which is hereby incorporated by reference, have a support layer that reflects the imaging radiation. In these constructions, the radiation from a laser pulse that passes through the imaging layer is returned to augment the effective flux through that layer and thus the efficiency of the imaging process.
While these constructions concentrate radiation within the imaging layer, they do not affect the efficiency with which radiation is distributed in that layer. Incident and reflected radiation follows the same, essentially straight-line path through the layer thickness. If, for example, the radiation-absorptive material is unevenly dispersed throughout the imaging layer, largely transparent regions will receive the same amount of radiation as densely absorptive regions, with consequent waste of laser power.
The present invention is directed to lithographic plate constructions having imaging layers that increase the distribution of imaging radiation within those layers, thereby improving the efficiency with which laser power is utilized. The present invention is also directed to methods of imaging lithographic plate constructions having such layers.
The present invention exploits the combination of a radiation-scattering material and a radiation-absorbing material dispersed in the imaging layer to increase the overall absorption of radiation in that layer. The radiation-scattering material may be in particulate form such that the particles reflect the radiation from their surfaces. Alternatively, the particles may scatter radiation through other optical properties such as, for example, diffraction. The presence of these particles dispersed within the imaging layer creates a large number of surfaces that reflect incident radiation at many angles throughout that layer. The overall effect is the scattering of radiation within the imaging layer. The radiation-absorbing material, also dispersed in the imaging layer, is thus exposed to incident radiation from the radiation source as well as the scattered radiation from within the imaging layer. This increased exposure of the radiation-absorbing material increases the efficiency and speed of ablation of the imaging layer.
The use of particles for the radiation-scattering material also provides beneficial porosity to the imaging layer. This porosity enhances adhesion to the overlying and/or the underlying layer and increases radiation penetration within the imaging layer.
The plates of the present invention can be either xe2x80x9cpositive-workingxe2x80x9d or xe2x80x9cnegative-working.xe2x80x9d In positive-working versions, areas that are inherently ink-receptive receive laser output and are removed, revealing a hydrophilic (or oleophobic) layer that will repel ink during printing; accordingly, the image area is selectively removed to reveal the background. In negative-working versions, areas that are inherently hydrophilic (or oleophobic) are removed to reveal an underlying ink-receptive layer, such that the exposed area forms the image and the unremoved top layer forms the background. An especially preferred construction is a xe2x80x9cdryxe2x80x9d plate with a silicone or fluorocarbon topcoat.
The radiation-scattering and radiation-absorbing materials may be in the form of powders, aggregates, or dyes, and are dispersed within the imaging layer. (As used herein, the terms xe2x80x9cdispersedxe2x80x9d and xe2x80x9cdispersionxe2x80x9d refer to any form of distribution within the imaging layer, e.g.,. conventional colloidal dispersions or suspensions of particles, solutions of a dye, distribution of material through codeposition as described below, etc.) The dispersion of these materials is immobilized through the use of a polymeric imaging layer, which may optionally be crosslinked in order to improve performance.
Alternatively, the imaging layer may be built up by deposit of polymer precursors, in which case the radiation-scattering and/or radiation-absorbing materials may be codeposited therewith. This approach facilitates creation of graded layers in which the concentration of radiation-scattering and/or radiation-absorbing material varies through the thickness of the layer, or is concentrated in one or more interlayers.
In one embodiment, the radiation-scattering material is a metal oxide, such as titanium oxide, tin dioxide, or zirconium oxide. In another embodiment, the radiation-scattering material comprises metallic particles (e.g., titanium, aluminum, magnesium, or other suitable metal).